A minigame that is not filler
Teaching Is Hard has you grading papers after class. On paper that is a chore mechanic — read the answer, pick a grade, move on. It exists because it is the delivery mechanism for the entire mystery, and it works precisely because it looks like filler.
The paper changes
One student's work drifts across the nights. Day one is ordinary handwriting and ordinary answers. By day three there is a small heart in the margin. Day five reads "sensei you smell nice". Day seven: "see you tonight sensei". You are grading her confessions in order, assigning them letters, and working out who she is by reading the escalation in her own handwriting.
Why not just tell the player
Because the game is about denial. If the reveal arrives as a cutscene, the player receives it. If it arrives as a note in a margin they graded and moved past, the player participates in ignoring it. That is a different feeling, and it is the one the story needs.
The cracks are scheduled
Around the grading sits a fixed schedule of intrusions. Day three she says something in class you convince yourself you misheard. Day five, a hair on the desk that is not yours. Day seven, lipstick. Day eight, she repeats something a student said in class that same day. Day ten she is still there for half a second after you wake. Day twelve the explanation collapses. None of it is random, because the tension depends on accumulation rather than surprise.
No content gating
One rule shaped everything: no player should miss a scene for choosing wrong. Choices determine which woman you see tonight, not whether you ever see her — the other returns within one to three nights, and roughly one night in four is quiet so the story can move. Replaying changes order, her mood toward you, and the late-game ending. Supporters paid for the content; hiding it behind a wrong turn would be a bad trade.